retro-imager/dependencies/libarchive-3.5.2/doc/wiki/ManPageArchiveRead3.wiki

ARCHIVE_READ(3) manual page 
== NAME == 
'''archive_read''' 
- functions for reading streaming archives 
== LIBRARY == 
Streaming Archive Library (libarchive, -larchive) 
== SYNOPSIS == 
'''<nowiki>#include <archive.h></nowiki>''' 
== DESCRIPTION == 
These functions provide a complete API for reading streaming archives. 
The general process is to first create the 
'''struct archive''' 
object, set options, initialize the reader, iterate over the archive 
headers and associated data, then close the archive and release all 
resources. 
=== Create archive object=== 
See 
[[ManPageArchiveReadNew3]]. 

To read an archive, you must first obtain an initialized 
'''struct archive''' 
object from 
'''archive_read_new'''(). 
=== Enable filters and formats=== 
See 
[[ManPageArchiveReadFilter3]] 
and 
[[ManPageArchiveReadFormat3]]. 

You can then modify this object for the desired operations with the 
various 
'''archive_read_set_XXX'''() 
and 
'''archive_read_support_XXX'''() 
functions. 
In particular, you will need to invoke appropriate 
'''archive_read_support_XXX'''() 
functions to enable the corresponding compression and format 
support. 
Note that these latter functions perform two distinct operations: 
they cause the corresponding support code to be linked into your 
program, and they enable the corresponding auto-detect code. 
Unless you have specific constraints, you will generally want 
to invoke 
'''archive_read_support_filter_all'''() 
and 
'''archive_read_support_format_all'''() 
to enable auto-detect for all formats and compression types 
currently supported by the library. 
=== Set options=== 
See 
[[ManPageArchiveReadSetOptions3]]. 
=== Open archive=== 
See 
[[ManPageArchiveReadOpen3]]. 

Once you have prepared the 
'''struct archive''' 
object, you call 
'''archive_read_open'''() 
to actually open the archive and prepare it for reading. 
There are several variants of this function; 
the most basic expects you to provide pointers to several 
functions that can provide blocks of bytes from the archive. 
There are convenience forms that allow you to 
specify a filename, file descriptor, 
''FILE *'' 
object, or a block of memory from which to read the archive data. 
Note that the core library makes no assumptions about the 
size of the blocks read; 
callback functions are free to read whatever block size is 
most appropriate for the medium. 
=== Consume archive=== 
See 
[[ManPageArchiveReadHeader3]], 
[[ManPageArchiveReadData3]] 
and 
[[ManPageArchiveReadExtract3]]. 

Each archive entry consists of a header followed by a certain 
amount of data. 
You can obtain the next header with 
'''archive_read_next_header'''(), 
which returns a pointer to an 
'''struct archive_entry''' 
structure with information about the current archive element. 
If the entry is a regular file, then the header will be followed 
by the file data. 
You can use 
'''archive_read_data'''() 
(which works much like the 
[[read(2)|http://www.freebsd.org/cgi/man.cgi?query=read&sektion=2]] 
system call) 
to read this data from the archive, or 
'''archive_read_data_block'''() 
which provides a slightly more efficient interface. 
You may prefer to use the higher-level 
'''archive_read_data_skip'''(), 
which reads and discards the data for this entry, 
'''archive_read_data_into_fd'''(), 
which copies the data to the provided file descriptor, or 
'''archive_read_extract'''(), 
which recreates the specified entry on disk and copies data 
from the archive. 
In particular, note that 
'''archive_read_extract'''() 
uses the 
'''struct archive_entry''' 
structure that you provide it, which may differ from the 
entry just read from the archive. 
In particular, many applications will want to override the 
pathname, file permissions, or ownership. 
=== Release resources=== 
See 
[[ManPageArchiveReadFree3]]. 

Once you have finished reading data from the archive, you 
should call 
'''archive_read_close'''() 
to close the archive, then call 
'''archive_read_free'''() 
to release all resources, including all memory allocated by the library. 
== EXAMPLES == 
The following illustrates basic usage of the library. 
In this example, 
the callback functions are simply wrappers around the standard 
[[open(2)|http://www.freebsd.org/cgi/man.cgi?query=open&sektion=2]], 
[[read(2)|http://www.freebsd.org/cgi/man.cgi?query=read&sektion=2]], 
and 
[[close(2)|http://www.freebsd.org/cgi/man.cgi?query=close&sektion=2]] 
system calls. 
```text
void
list_archive(const char *name)
{
  struct mydata *mydata;
  struct archive *a;
  struct archive_entry *entry;
  mydata = malloc(sizeof(struct mydata));
  a = archive_read_new();
  mydata->name = name;
  archive_read_support_filter_all(a);
  archive_read_support_format_all(a);
  archive_read_open(a, mydata, myopen, myread, myclose);
  while (archive_read_next_header(a, &entry) == ARCHIVE_OK) {
    printf("%s\en",archive_entry_pathname(entry));
    archive_read_data_skip(a);
  }
  archive_read_free(a);
  free(mydata);
}
la_ssize_t
myread(struct archive *a, void *client_data, const void **buff)
{
  struct mydata *mydata = client_data;
  *buff = mydata->buff;
  return (read(mydata->fd, mydata->buff, 10240));
}
int
myopen(struct archive *a, void *client_data)
{
  struct mydata *mydata = client_data;
  mydata->fd = open(mydata->name, O_RDONLY);
  return (mydata->fd >= 0 ? ARCHIVE_OK : ARCHIVE_FATAL);
}
int
myclose(struct archive *a, void *client_data)
{
  struct mydata *mydata = client_data;
  if (mydata->fd > 0)
    close(mydata->fd);
  return (ARCHIVE_OK);
}
```
== SEE ALSO == 
[[ManPageBsdtar1]], 
[[ManPageArchiveReadData3]], 
[[ManPageArchiveReadExtract3]], 
[[ManPageArchiveReadFilter3]], 
[[ManPageArchiveReadFormat3]], 
[[ManPageArchiveReadHeader3]], 
[[ManPageArchiveReadNew3]], 
[[ManPageArchiveReadOpen3]], 
[[ManPageArchiveReadSetOptions3]], 
[[ManPageArchiveUtil3]], 
[[ManPageLibarchive3]], 
[[ManPageTar5]] 
== HISTORY == 
The 
'''libarchive''' 
library first appeared in 
FreeBSD 5.3. 
== AUTHORS == 
The 
'''libarchive''' 
library was written by 
Tim Kientzle  &lt;kientzle@acm.org.&gt; 
== BUGS == 
Many traditional archiver programs treat 
empty files as valid empty archives. 
For example, many implementations of 
[[ManPageBsdtar1]] 
allow you to append entries to an empty file. 
Of course, it is impossible to determine the format of an empty file 
by inspecting the contents, so this library treats empty files as 
having a special 
"empty" 
format.